Abstract Brain cancer is a terrifying diagnosis representing a relatively large segment of childhood cancer, yet thanks to great advances in treatment, survival rates among children now exceed 80%. These positive clinical outcomes require the use of radiotherapy (RT), but like other treatment modalities, RT causes significant long- term neurocognitive sequelae impacting not only cancer survivors, but also their caregiver networks and society. Adults receiving RT for brain cancers also suffer similar symptoms and would benefit from the amelioration or elimination of the neurocognitive effects of RT. In theory, RT-induced brain injury is easier to treat than other brain injuries, given that the time of injury is known and pretreatment is feasible. However, the unclear nature of RT-induced brain damage is a major obstacle to doing so. Research on RT-induced brain injury has focused on the dividing neuroprogenitor (NP) cells from which a small pool of postnatal, ?adult- born? neurons arises in the dentate gyrus, as the hippocampus (of which the dentate gyrus is part) is particularly sensitive to radiation. Certainly, NP cell damage contributes to RT-induced sequelae. However, we and others have recently shown that terminally differentiated neurons, long thought to be resistant to radiation, undergo synaptic alterations in response to radiation. This observation has major implications for the treatment of RT-induced sequelae because it suggests that the damage could happen throughout the entire brain and not be limited to the small, discrete sites of postnatal neuron formation. In this proposal, we present our most recent data on this phenomenon, showing that therapeutic doses of radiation lead to ectopic synaptogenesis and synapse potentiation within 1 hr of RT. Females are more affected than males by this insult, and suppressing glutamate signaling prevents both synapse expansion and subsequent long-term synapse loss. Many questions remain unanswered, however: How localized is the injury? Does the injury promulgate along neuronal circuits? Are some regions of the brain more or less susceptible to the injury? How are these parameters affected by the sex of the individual undergoing RT? Can RT-mediated synaptic defects be reversed? These are critical questions whose answers are required to rationally design therapies to combat RT-induced neurocognitive sequelae. We propose to define the nature of acute RT-induced synaptic damage at by (i) creating at atlas of RT-mediated synaptic injury in the mouse brain using fluorescent probes for neuronal activity and synaptic potentiation, advanced imaging techniques and multidimensional analysis, (ii) testing the influences of age and sex on subject response to RT, and (iii) manipulating cellular signaling to attempt to reverse RT-mediated synaptic damage. We propose a novel and multidisciplinary approach to dissecting the nature of RT-mediated synaptic damage at the molecular, synaptic, cellular, and organismal, i.e. behavioral, levels. The results of this translation proposal have the potential to greatly and positively impact the health of cancer survivors of all ages, and our techniques could be used to investigate other types of brain injuries.